View Full Version : Formula for focus distance vs. extension

28-Sep-2007, 12:45
With a normal, non-retrofocus or telephoto lens design, how does extension correlate to the distance you've focused?

For example, say you've got a 355mm G-claron. You extend your rail to 400mm. How close have you focused? What's the formula?

28-Sep-2007, 13:04
The formula is:

( 1/ focal length ) = (1 / image distace) + (1/object distance)

The image distance is, nominally the distance from the film plane to the front of the lensboard (not really but it's a good enough approximation in this case).

and the object distance is the distance from the front of the lensboard to the plane of focus.

28-Sep-2007, 13:06
oh, so in your example,

( 1/ 355 ) = (1 / 400 ) + 1/x

(1/355) - (1/400) = 1/x

x = 1 / (( 1/355) - (1/400))

I get an object distance of about 3.15 meters.

Helen Bach
29-Sep-2007, 07:51
Has that answered your question, or do you need more information?


Arne Norris
29-Sep-2007, 08:34
Tom Westbrook gave a good formula on this photo.net page:


One question that I would love to verify is how to calculate for telephoto lenses.

Helen Bach
29-Sep-2007, 13:38
One question that I would love to verify is how to calculate for telephoto lenses.

The formula is fundamentally the same as the one Brad gave - it works for all lenses if you measure from the relevant nodal points instead of the lens board, as alluded to by Brad. For a telephoto you could do it by focusing on infinity and measuring the bellows extension, then working out the distance from the lens board to the rear (second) nodal point. Then apply that to the equation. On the other hand you might be able to get the information from the manufacturer.

Here is an example for a 400 mm lens.

You focus on infinity, and measure the distance from the film plane to the lens board as being 285 mm. Assuming that the lens has a true focal length that is close to 400 mm, the rear nodal point is 115 mm in front of the lens board. This will be constant. All you do now is to replace 'image distance' in the formula that Brad gave with 'bellows draw plus 115 mm'. This would be complicated further by close subjects (closer than about ten times the focal length, depending on the accuracy required) because the object distance should be measured from the front (first) nodal point.

Does that help?


Ben R
29-Sep-2007, 14:10
No doubt the formula would be as efficient for doing the actual focusing where GG isn't an option or you can't use a coupled rangefinder. You could carry a chart and use one of those golf type laser rangefinders which would have the accuracy for LF.

1-Oct-2007, 11:34
What's this? still, no reply from the original poster?

I guess it wasn't too important. :(

1-Oct-2007, 12:48
Sorry; I appreciate the answers. It helps. I posted this on apug as well and got an answer there so I hadn't checked this thread.

This is for a planned camera build - we're figuring out the maximum & minimum extensions we'll want based on the lenses we're likely to be using. So getting a rough estimate of this is very important, though exact numbers down to the millimeter aren't really necessary.

Thanks ;)